1. Field of the Invention
This invention relates to a repeater selection system for identifying and selecting a desired one of a plurality of repeaters in a digital signal transmission system.
This invention further relates to a monitor system for a PCM code or like digital signal relay system which monitors the characteristic of the relay system and detects and locates faults in a repeater and in the transmission line section between adjacent repeaters.
2. Description of the Prior Art
In a digital signal transmission line system, there have heretofore been employed for monitoring repeaters or the fault location a pulse trio system (refer to Nippon Telegraph and Telephone Public Corporation, Reports of Studies and Practical Applications, Vol. 14, No. 1), a phase detection system (refer to Nippon Telegraph and Telephone Public Corporation, Reports of Studies and Practical Applications, Vol. 25, No. 1), a bipolar check system (refer to the Institute of Electronics and Communication Engineers of Japan, the Society for the Study of Communication Systems, data CS-72-156), etc. All of these systems are of the type employing a monitoring line, called an interstitial copper pair, for the transmission of a repeater identify signal or a signal turned back from a repeater, and adopting a method of discriminating repeaters in terms of frequencies, that is, a method which uses a band-pass filter of a frequency peculiar to each repeater and provides on the transmission line a signal having a code configuration of high energy density at the frequency of the band-pass filter for a desired repeater to identify or select the repeater by an increase in the output level of the band-pass filter. In a long distance digital transmission system, however, the use of the interstitial copper pair is disadvantageous for the following reasons:
(1) A long transmission distance necessitates repeaters in the interstitial copper pair, resulting in complexity in the system arrangement.
(2) The reliability of the interstitial copper pair transmission system must be designed very high, which is uneconomical from the standpoint of the design of the entire system.
(3) The use of the interstitial copper pair introduces complexity in the cable structure. Therefore, a system which monitors the transmission system by utilizing a main transmission line without employing the interstitial copper pair is very effective from the viewpoints of simplification and economization of the entire transmission system.
In an ordinary land digital signal transmission line system, use is made of an arrangement in which a plurality of pairs of backward and forward transmission lines are housed in one cable and regeneration is achieved by regenerative repeater circuits for the forward and backward directions disposed in the same repeater housing. With such an arrangement, it is possible to provide an additional circuit in each repeater and to couple the backward and forward regenerators to each other via the additional circuit. For an optical fiber transmission system employing an optical fiber as a transmission line, there has been proposed a system in which a signal in the backward transmission line is sent back to the forward transmission line via a switch in an electric circuit of an optical repeater (refer to Japanese Pat. No. 99904/76). This system does not employ the aforesaid interstitial copper pair but, for selecting a desired repeater and controlling the switch, uses a method of identifying repeaters in terms of frequencies, which is similar to that adopted in the past; therefore this method has the following detects:
(1) Since a select signal to be applied to the transmission line has a high energy density at each of a clock frequency and the center frequency of the band-pass filter of a desired repeater, severe limitations are imposed on the code configuration.
(2) This leads to a difficulty in the measurement of variations in the error rate due to a modification of the code configuration in the testing of the transmission line turned back from each repeater.
(3) Because of the availability of the performance of narrow band-pass filters and the degree of energy concentration on a particular frequency depending on the code configuration, it is difficult to reduce the intervals between adjacent frequencies assigned to the repeaters, which presents a problem in a long repeatered system.
On the other hand, a land coaxial PCM relay system usually employs an arrangement in which several pairs of coaxial cables are housed as backward and forward transmission lines in one cable and regeneration in each transmission line is achieved by each of regenerators disposed in the same repeater housing. A submarine cable system employing optical fibers also adopts a similar arrangement in which several pairs of optical fibers are housed as backward and forward transmission lines in one cable and regeneration in each transmission line is effected by regenerators for the forward and backward transmission directions disposed in the same repeater housing. With such an arrangement, it is possible to provide an additional circuit in each repeater and couple regenerators to the backward and the forward transmission via the additional circuit. In the land coaxial PCM relay system, there is no precedent for this kind of monitoring system; but in an optical fiber transmission system, there has been proposed a system in which a signal in the backward transmission line is sent back to the forward transmission line via a switch in an electric circuit of an optical repeater (refer to Japanese Pat. No. 99904/76). But this method has such defects as follows:
(1) If a switch is inserted in a main transmission line, the reliability of the main transmission line system is markedly degraded.
(2) If a monitor signal composed mainly of repeater-identifying and selecting information is sent back as it is, a severe limitation is imposed on a test pattern of a repeatered line.
(3) A code error occurring in the backward transmission line and a code error in the forward transmission line cannot be separated.